The modal verbs can/could, may/might, must, shall/should, will/would always take bare infinitive complements: infinitives without the to marker. This doesn't change when you place an adverbial between the modal and its infinitive complement:
I would say ...
I would more likely say ...
There are however, other problems with your rewrite.
First, the original sentence is unidiomatic. We almost never use of phrases to express the genitive of personal pronouns.
If I were in your place I would tell him to stop crying.
Second, you cannot simply replace tell with say, for two reasons:
tell may be "ditransitive", taking both a Direct and an Indirect Object, but say is "monotransitive", taking only a Direct Object. With say the person addressed must be expressed with a to preposition phrase.
I told this to him or I told him this.
I said this to him but not *I said him this.
tell may take a marked infinitival clause (to DO something) in place of a nominal Direct Object. In this case the Indirect Object is taken to be the subject of the infinitival clause, and tell is understood to have the sense "command" or "instruct":
I told him to stop crying = I commanded him that he must stop crying.
say cannot be used this way; it does not take infinitival complements.
*I said him to stop crying.
And say does not have the sense command; this can be expressed only indirectly, by employing a that clause which includes a term which has the sense command:
I said to him that he must stop crying.